* Prometheus data source: Update tooltip for _Scrape interval_

The old tool tip was plainly wrong. The globally configured scrape
interval is not necessarily the most used scrape interval, so the new
wording is describing what actually matters. The evaluation interval
wasn't even mentioned before. And finally, the configured value is
not strictly a lower limit for the step query parameter as it can be
overridden by the panel option _Min time interval_.

I plan to explain the overriding intricacies in the tool tip for _Min
time interval_.

Signed-off-by: beorn7 <beorn@grafana.com>

* Improve tooltip for _Min step_

The previous tool tip was mostly addressing aspects that are not
specific to _Min step_ (and might be more appropriate to be added to
the _Min time interval_ tool tip as that setting has a wider
scope). The new version emphasizes the important gotchas: that this is
an _additional_ lower limit, and that it is _not_ multiplied by the
resolution factor.

Signed-off-by: beorn7 <beorn@grafana.com>

* Fixed snapshot

Co-authored-by: David <david.kaltschmidt@gmail.com>